The use of concrete as a building material is widely known as is its fundamental strength in compression and weakness in tension. It is very desirable in many construction applications to utilize materials which can withstand both compressive and tensile forces. As concrete is typically unable to resist tensile forces, some type of tensile reinforcement must be utilized with the concrete. It has been traditional to utilize deformed steel bars, commonly called "rebar", for this purpose, particularly for reinforcing concrete slabs. When combined with concrete, the rebar enables concrete to function as a construction material capable of resisting both tensile and compressive forces.
Pre-stressed concrete, which is a logical extension of the traditional rebar-reinforced concrete, utilizes reinforcement by high strength steel which is pre-stressed within the concrete thereby providing active tensile reinforcement within the concrete versus the passive reinforcement which resulted with the traditional, rebar-reinforced concrete. As is well known to those of skill in the art, such active reinforcement has been found to dramatically extend the range of applications where concrete can be used.
Pre-stressed concrete systems can be divided into two (2) basic types which are pre-tensioning pre-stressing systems and post-tensioning pre-stressing systems. In pre-tensioning systems, high strength steel strands, often referred to as "tendons", are bonded to concrete and caused to be tensioned prior to hardening of the concrete. As is known to those of skill in the art of pre-stressed concrete, it is very difficult to build continuous structures with pre-tensioned concrete.
Post-tensioning pre-stressing systems can be divided into bonded and unbonded systems. In both bonded and unbonded post-tensioning systems, a tendon comprises one or more strands, and each strand typically comprises a plurality of high-strength steel wires. The strands are positioned within a duct that is within concrete, and each strand is stressed (placed under tension) after the concrete has hardened or cured. In bonded post-tensioning systems, the duct encapsulating the strands is filled by injection with grout to bond or lock the strands in position within the duct. In unbonded post-tensioning systems, no grout is placed within the duct to surround and lock the strands in position. Rather, the strands are lubricated with grease and maintained or locked in position only by the tension resulting from the attachment of opposing ends of the strands to opposed anchorages. With unbonded post-tensioning systems, a loss of function of one or both anchorages, such as from fire, physical damage, or even corrosion, typically causes the entire pre-stressing force for each strand to be lost since the strands are not bonded to the concrete. Today, the use of supplemental reinforcing rebar is therefore required with unbonded post-tensioning systems to provide redundancy for the post-tensioning system. Quite distinguishably, bonded post-tensioning systems enable each strand to be locked or bonded continuously along its length and typically will not allow the strand pre-stressing force in each strand to be lost for its entire length because of a localized problem with the pre-stressing force of the strand, such as that caused by partial or complete loss of an anchorage.
The early construction applications for pre-stressing, post-tensioning systems utilized bonded post-tensioning systems which is considered to be the more traditional method of applying pre-stressing to concrete. In the 1960's the use of pre-stressed concrete was adapted for thin concrete slabs for use as a structural system, for example in buildings and parking garages. It was for this application that single strand unbonded post-tensioning systems, commonly referred to as "mono-strand systems" were developed wherein the single strand tendons were first coated with grease and then encapsulated with plastic. A potential disadvantage which can result from the use of mono-strand systems is possible corrosion resulting from deicing salts used during winter. This can particularly occur in parking garages where cars carry the deicing salts in with the ice and snow, and the deicing salts melt and enter the concrete through cracks, thereby allowing the deicing salts to attack and corrode the pre-stressing wires of the strand. It is generally recognized now that some deficiencies exist in such mono-strand systems, and such deficiencies can be acute at intermediate construction joints in the mono-strand systems. At such construction joints, it is necessary to strip the plastic encapsulation from the unbonded strands for the purpose of stressing and making the connection with intermediate anchorages. The intermediate construction joints also provide a logical crack or break in the concrete and have been known to provide a passage through which corrosive elements, such as salt laden water, have entered the concrete and caused corrosion of the mono-strand system.
It is therefore desirable and advantageous to utilize bonded post-tensioning systems for pre-stressing concrete used as a construction material. When utilizing bonded post-tensioning concrete systems, tendon sections are often interconnected in series by an intermediate anchorage, sometimes referred to as a coupler, which is typically used at intermediate construction joints. To provide means for inserting grout by injection into the duct of each such tendon section, existing bonded post-tensioning systems utilize a grout vent for each tendon section interconnected in series with each grout vent extending through the concrete from the duct within the concrete to an exterior area of the concrete which is usually through the top surface of the concrete slab or beam. Quite understandably, such grout vents are a tremendous hinderance for the concrete finishing operation. Furthermore, such grout vents can serve as pathways for corrosive elements, such as salt laden water, to easily access the pre-stressing ducts and grouted strands contained therein to possibly corrode the strands, particularly if proper sealing of the grout vents did not occur. Yet another problem with interconnected bonded post-tensioning systems is that the connection between the anchorages and the ducts are usually not sealed with anything other than tape or a friction-type seal, and the connection therefore will not provide an adequate water-tight seal.
In light of the existing post-tensioning systems, there remains much room for improvement in the art for an improved bonded post-tensioning apparatus and method.